Bimetallic thermometer



April 26, 1955 E. R. KEBBN 2,705,962

BIMETALLIC THERMOMETER Filed Dec. 7, 1951 I l l EARL l?. KEBBO/V INVENTOR.

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nited States Patent Glitice 2,706,962 Patented Apr. 26, 1955 2,706,962 BIIVIETALLIC THERMOMETER Earl R. Kebbon, Chatham, N. J., assigner to Weston Electrical Instrument Corporation, Newark, N. J., a corporation of New Jersey Application December 7, 1951, Serial N o. 260,432

3 Claims. (Ci. 116-129) This invention relates to bimetallic thermometers and more particularly to a precision thermometer including one or more auxiliary pointers for indicating the maximum excursions of the main indicating pointer and a selectively-operable resetting arm for restoring such auxiliary pointers to a desired position relative to the instrument scale.

The bimetallic element of a sensitive thermometer comprises a thin ribbon of bimetal wound in the form of a helical coil of one or more layers. One end of the coil is anchored to a suitable part of the thermometer housing and the other end is attached to a suitable indicating pointer which cooperates with a scale calibrated in temperature values. Temperature changes at the bimetallic coil result in a rotation of the indicating pointthe appended claims.

In the drawings wherein like reference characters denote like parts in the several views:

Figure 1 is a front view of a thermometer made in accordance with this invention;

Figure 2 is a side elevation of the device shown in Figure 1 with part of the stem broken away;

Figure 3 is a fragmentary, central cross-sectional view of the device, drawn to an enlarged scale; and

Figure 4 is an isometric view showing the construction of the maximum and minimum pointers and the reset arm.

Referring now to Figures l and 2, the thermometer comprises a tubular stem 10 in which the helical, bimetallic coil 11 is mounted. As shown, one end of the coil is secured to a plug 12, as by soldering or welding, soldered or brazed to the shell to close indicating pointer 15 that temperature values visible 17 of the instrument ff is a main co-operates with a scale 16 of through the transparent cover When the instrument is designed provided with a threaded, hexagonal nut 18 that serves mark. Consequently, the at any given time, the minimum temperature to which the bimetallic coil has been subjected. Similarly, the auxiliary pointer 21 will indicate, at

T bent upwardly for engagement with It will be noted of the auxiliary pointers have radial reset arm R whereby the rotated freely, by means of the knob 25, upstanding end T striking the tail of either strument. Variable such restraining forces will further effect the reliability of the indications. Still further, the

resetting arm must transmit sutiicient force to either or is shown as comprising a base 26, the transparent cover the numeral 30, and the flat nut 31 cooperates with the outer nut 18 to clamp the stern to the base 26, suitable washers 32, 33 being provided to retain the parts in fixed position. The staff 13 passes through a bushing 34 having a tapered inner section that serves as a bearing to limit lateral movement of the staff to which the main indicating pointer 1S is attached as by the bushing 36 and spring washer 37. As shown, the pointer 1S is spaced from the scale plate 3S that is secured to the base 26 by the screws 39, and the pointer carries an upwardly-extending merriber or vane 24. The auxiliary pointer and resetting arm assembly comprises a shaft 40 to which the external knob 25 is secured by a set screw 41. A central bore in the knob accommodates the flat nut 42 which is threaded on the bushing 43 extending through a central hole in the transparent cover 17. It will be noted that the bushing 43 has a base of enlarged diameter which seats against the inner surface of the cover whereby the opening in the cover is sealed effectively, when the nut 42 is in place over the fiat washer 44. It may here be pointed out that the bushing 43 and the nut 42 remain in fixed position relative to the cover. The shaft 40 passes through an axial hole in the bushing 43 said hole terminating in an enlarged-diameter bore to accommodate the shank of the hollow bushing 45. The shaft 40 includes an integral, circular fiange 46 terminating in a reduced-diameter section 47 that passes through a central hole in the fiat washer 48. Each of the auxiliary pointers 20, 21 and the reset arm R are provided with circular hubs having central holes therein, as shown in Figure 4, the respective hubs being identified by the numerals 50, 51, S2.

The hub section of the auxiliary pointers and reset arm are fiat and the hole in each has a diameter slightly larger than the diameter of the bushing element that is inserted therethrough in the assembly of the mechanism. Thus, as shown in Figure 3, the hub S2 of the reset arm R, is positioned between the circular fiange 46 (of the shaft 40) and the fiat washer 4S when the end 47 of the shaft is peaned over, as shown. It will be noted that the hub 52, of the reset arm, is disposed over a circular shoulder extending from the fiange 46. The axial height of such shoulder is slightly greater than the thickness of the reset hub from which it will be apparent that the reset arm is free to rotate relative to the shaft 40. The surfaces of the hub 52 and the contacting surfaces of the washer 48 and of the flange 46 are coated with a thin film of suitable grease thereby providing a frictional assembly whereby the reset arm R will rotate, upon rotation of the shaft 40. When the upwardly-extending end T, of the reset arrn, strikes the forward end of one or both of the auxiliary pointers 20, 21 these pointers will rotate therewith. However, when the reset arm is even gently restrained against rotational movement, continued rotation of the shaft merely results in slippage between the reset arm hub and the cooperating friction surfaces. I have found that the very thin film of silicone grease applied to relatively large area, fiat contacting surfaces permits of a relatively loose assembly between the reset arm and the friction surfaces provided by the washer 48 and the fiange 46 of the shaft. When the knob 25 is rotated slowly, the silicone grease provides frictional engagement between the shaft and the reset arm to impart rotary movement to the auxiliary pointers. A sudden rotation of the knob at the moment of contact between the reset arm and the auxiliary pointerI results in slippage with little or no motion imparted to the pointer.

The upper auxiliary pointer 21 is disposed over the bushing 43 and is held in position by the fiat washer 54. The lower auxiliary pointer 20 is disposed over the bushing 45 and the pointer hub abuts against the fiat washer S4 when the bushing 45 is force-fitted into the enlarged bore of the bushing 43. It will be noted that the hub section of each auxiliary pointer is fitted over a reduced diameter boss, or shoulder, on the cooperating bushing. The axial length of each boss is slightly greater than the thickness of the associated hub section so that there is no mechanical clamping of the pointer hubs by the cooperating surfaces of the bushings or the fiat washer. All such cooperating surfaces are coated with a thin film of silcone grease. Thus, when the vane 24, carried by the main indicating pointer 15, engages the auxiliary pointer vane 23 or 22, the auxiliary pointer will rotate with no appreciable load added to the relatively-slow, motional response of the temperature-sensitive bimetallic coil. However, the frictional restraint provided by the grease is sufficient to retain the auxiliary pointers in any given position relative the instrument scale regardless of the angular disposition of the instrument as a whole. It will be noted that the effective surface area of the grease film associated with the relatively movable surfaces of the reset arm is greater than that associated with either of the auxiliary pointers. Such larger, surface area of the grease film provides the reset mechanism with sufficient friction to rotate the maximum and/or minimum pointers against the latters own friction. Although I prefer to coat the relatively slidable surfaces with a silicone grease, a silicone oil is satisfactory for the purpose when the thermometer is not subjected to vibrations. Also, grease other than silicone may be used on thermometers that are not subjected to wide temperature variations.

While I have illustrated my novel construction and assembly of maximum and minimum indicators and a selectively-operable 4reset as applied to a relatively large industrial bimetallic thermometer, I have successfully applied such construction to a pocket type birnetallic themometer having a one inch dial and a scale range of -40/160 degrees Fahrenheit. l am aware of prior grease type frictional mechanisms but such prior arrangements include a relatively large disc rotatable within a relatively large mass of grease retained within a cup-like reservoir. While such arrangements are suitable for use with devices generating a relatively large driving torque they are not adapted for use in a sensitive thermometer wherein the bimetallic coil generates only a feeble rotational torque.

Those skilled in this art will understand that the vanes carried by the main and auxiliary pointers constitute a preferred form of the invention. Such vanes may be omitted and proper operation obtained by bending the tip of the main pointer upward to exten into the plane of the upper auxiliary pointer 21.

Having new described my invention what I desire to protect by United States Letters Patent is set forth in the following claims.

I claim:

l. A maximum and minimum indicator and reset mechanism for a bimetallic thermometer of the type comprising a rotatable main pointer visible through a transparent cover closing the thermometer housing, said mechanism comprising a hollow bushing extending through a central opening in the cover, said bushing being secured in fixed position relative to the cover; a shaft extending through the bushing and including an enlarged-diameter flange disposed within the housing and spaced from the inner end of the said bushing; a pair of auxiliary pointers rotatably carried by the shaft, said pointers adapted to be engaged by the main pointer and rotated about the shaft', a reset arm having one end offset to engage each of the auxiliary pointers and the other end pivotally secured to the said fiange portion of the shaft; a thin film of silicone grease between the co-acting surfaces of the reset arm and ange portion; and a finger-operable knob external of the cover and secured to the said shaft.

2. A maximum and minimum indicator and reset mechanism for a bimetallic thermometer of the type comprising a rotatable main pointer visible through a transparent cover closing the thermometer housing, said mechanism comprising a first hollow bushing having a head abutting the inner surface of the cover and a shank passing through a central hole in the cover; means securing the said bushing in fixed position relafirst indicator having a fiat hub including a hole, one surface of said hub abutting the head of the said first bushing; a fiat washer abutting the other surface of said hub; a second indicator having a fiat hub including a hole, one surface of said hub abutting the said fiat washer', a second hollow bushing having a head abutting the other surface of the hub of the secondv indicator and a shank force-fitted into the said first hollow bushing, said second bushing securing the two indicators and fiat washer in loose assembled relation such that either indicator is freely rotatable; a thin film of silicone grease on the hub of each indicator and serving as a frictional coupling tending to prevent rotation of the indicators; a shaft extending through the first and second hollow bushings, said shaft having an enlarged head abutting the head of the second hollow bushing', a finger-operable knob lsecured to the outer end of the shaft; a reset arm having one end offset to engage each of the indicators and a hole in the other end, the latter end abutting the head of clude at contiguous surfaces with a thin film of silisaid shaft; a second ilat washer disposed over the cone grease between the said contiguous surfaces. said end of the reset arm; means loosely securing the scond washer tche shaft; a thin film of silicone grease on 5 References Cited in the le of this patent t e surfaces of e said other end of the reset arm and tending to prevent rotation of the reset arm relative UNITED STATES PATENTS to the shaft; and means carried by the main pointer 1,018,582 Nelson Feb. 27, 1912 for individually engaging the indicators to impart rota- 2,457,286 Tollefsen et al. Dec. 28, 1948 tion thereto. 2,518,928 Paine et al. Aug. 15, 1950 3. In a birnetallic thermometer of the type compris- 10 2,558,743 Ford July 3, 1951 ing a temperature-sensitive coil actuating a main pointer 2,577,492 Teter Dec. 4, 1951 over a scale, an auxiliary pointer adapted for rotation by the main pointer and a reset arm carried by FOREIGN PATENTS a rotatable shaft that extends externally of the ther- 172,118 Switzerland Sept. 30, 1934 mometer housing, said reset arm being adapted upon 15 332,237 Great Britain July 10, 1939 rotation to engage the said auxiliary pointer; the im- 993,338 France July 25, 195i provement wherein the said shaft and reset arm in- 

1. A MAXIMUM AND MINIMUM INDICATOR AND RESET MECHANISM FOR A BIMETALLIC THERMOMETER OF THE TYPE COMPRISING A ROTATABLE MAIN POINTER VISIBLE THROUGH A TRANSPARENT COVER CLOSING THE THERMOMETER HOUSING, SAID MECHANISM COMPRISING A HOLLOW BUSHING AND INCLUDING THROUGH A CENTRAL OPENING IN THE COVER, SAID BUSHING BEING SECURED IN FIXED POSITION RELATIVE TO THE COVER; A SHAFT EXTENDING THROUGH THE BUSHING AND INCLUDING AND ENLARGED-DIAMETER FLANGE DISPOSED WITHIN THE HOUSING AND SPACED FROM THE INNER END OF THE SAID BUSHING; A PAIR OF AUXILIARY POINTERS ROTATABLY CARRIED BY THE SHAFT, SAID POINTERS ADAPTED TO BE ENGAGED BY THE MAIN POINTER AND ROTATED ABOUT THE SHAFT; A RESET ARM HAVING ONE END OFFSET TO ENGAGE EACH OF THE AUXILIARY POINTERS AND THE OTHER END PIVOTALLY SECURED TO THE SAID FLANGE PORTION OF THE SHAFT; A THIN FILM OF SILICONE GREASE BETWEEN THE CO-ACTING SURFACES OF THE RESET ARM AND FLANGE PROTION; AND A FINGER-OPERABLE KNOB EXTERNAL OF THE COVER AND SECURED TO THE SAID SHAFT. 